The invention relates to a circuit arrangement for determining the impedance of a test battery having a first test battery pole of a first polarity and a second test battery pole of a second polarity. The circuit arrangement includes a first test connection for connecting the first test battery pole and a second test connection for connecting the second test battery pole, an AC source which is connected to the test connections and can be used to apply an alternating current to the test battery in the connected state, and a voltage recording device which is connected in parallel with the test connections and the AC source and can be used to record a voltage profile between the test connections.
The invention also relates to a method for determining the impedance of a test battery having a first test battery pole of a first polarity and a second test battery pole of a second polarity. The method includes the steps of:
a) applying an alternating current to the test battery by way of an AC source connected to its test battery poles, with the result that an accordingly modulated voltage results at the test battery poles,
b) recording the profile of the resulting voltage by way of a voltage recording device which is connected to the test battery poles of the test battery in parallel with the AC source and the test battery, and
c) determining the impedance of the test battery by comparing the profiles of the applied alternating current and of the resulting voltage.
Such a circuit arrangement and such a method as noted above are known from WO 2012/088555 A1. The impedance is an important property of a battery and can be used to determine the state of health of the battery, for example. In the known circuit arrangement, an AC source is connected to test connections to which a test battery can be connected. The circuit arrangement additionally has a voltmeter for measuring the resulting voltage profile between the test connections.
It is known that an alternating current applied to the poles of a battery generates a modulated voltage response, with the result that the impedance of the battery can be determined by means of a comparison, in particular by means of complex division. In practice, this is generally effected by determining the phase shift between the applied current and the resulting voltage signal and dividing their real amplitudes. For a more detailed characterization of the battery, measurements are usually carried out at different current or voltage frequencies and/or with more complicated excitation signals which differ from pure sinusoidal excitations, in which case the entire range of spectroscopic techniques is available to a person skilled in the art. Therefore, this is also referred to as impedance spectroscopy in this context.
The voltage profile recorded with the voltmeter is composed of a temporally constant DC component, namely the open-circuit voltage of the test battery, and an AC component which temporally varies on the basis of the alternating current applied. The parameters relevant to the determination of the impedance, such as the phase information, are able to be determined only from the AC component. At the same time, however, an exact determination of the AC component is made more difficult by the concomitant recording of the DC component which is large in comparison with the AC component, since it fills virtually the entire resolution range of the voltmeter.
The object of the present invention is to develop a circuit arrangement of the generic type in such a manner that the AC component contained in the voltage profile can be recorded with higher resolution.
This and other objects are achieved in accordance with the invention by virtue of the fact that the voltage recording device includes:                a voltage measuring unit for measuring a voltage profile between two measurement connections of the voltage measuring unit, and        a compensation voltage source which is connected in series with the voltage measuring unit, provides a DC voltage and has a first pole of the first polarity and a second pole of the second polarity, wherein        the first pole of the compensation voltage source is connected to the first test connection, the second pole of the compensation voltage source is connected to the first measurement connection, and the second measurement connection is connected to the second test connection.        
The object is also achieved in accordance with the invention by virtue of the fact that the voltage recording device includes:                a voltage measuring unit for measuring a voltage profile between two measurement connections of the voltage measuring unit, and        a compensation voltage source which is connected in series with the voltage measuring unit, provides a DC voltage and has a first pole of the first polarity and a second pole of the second polarity, wherein the first pole of the compensation voltage source is connected to the first test battery pole, the second pole of the compensation voltage source is connected to the first measurement connection, and the second measurement connection is connected to the second test battery pole, with the result that, when recording the profile of the resulting voltage, its DC component is at least partially compensated for by the DC voltage from the compensation voltage source.        
According to the invention, a DC counter-voltage which compensates for a corresponding DC component of the original measurement voltage is therefore connected in opposition to the original measurement voltage. In other words, the DC voltage offset of the original measurement voltage is eliminated, with the result that the resulting measurement voltage only consists of the AC component enforced by the alternating current being impressed and—in the event of incomplete compensation—a reduced residual amount of the DC component. The latter can be reduced completely to zero by adjusting the compensation voltage to the open-circuit voltage of the test battery.
The alternating current which is responsible for producing the AC component and is applied to the test battery can be different. For this purpose, a sinusoidal alternating current of a particular frequency can be used, for example. However, in connection with impedance spectroscopy, a person skilled in the art is also aware of a number of further possibilities. For example, the alternating current need not necessarily be sinusoidal. It is only important that the impedance of the test battery can be determined from the alternating current profile and the measured voltage profile. The alternating current profile can be measured, for example, using a current measuring unit connected in series between one of the test connections and the AC source. Alternatively, however, it goes without saying that it is also possible to read the alternating current profile directly from the AC source or from a control unit controlling the AC source.
One preferred embodiment of the invention provides for the DC voltage provided by the compensation voltage source to be equal to the open-circuit voltage of the battery within a predefined tolerance. This makes it possible to advantageously virtually completely compensate for the open-circuit voltage of the test battery and therefore to determine the AC component of the voltage profile with an extremely high resolution and therefore extremely accurately.
Another embodiment of the circuit arrangement according to the invention provides for the compensation voltage source to be in the form of a controllable DC voltage source. This embodiment makes it possible to advantageously use the circuit arrangement for test batteries having different open-circuit voltages. It is therefore not necessary to exchange the compensation voltage source in order to determine the impedance of a second test battery. For example, before the alternating current is applied, the open-circuit voltage of the test battery can be measured and a corresponding counter-voltage can be controlled at the controllable compensation voltage source.
In other words, a method is considered to be advantageous in which use is made of a controllable compensation voltage source which is controlled in such a manner that, before the profile of the voltage resulting from the alternating current applied to the test battery is recorded, it provides a DC voltage which is equal to the open-circuit voltage of the test battery within a predefined tolerance. The controllable compensation voltage source may be implemented, for example, in the form of a digital/analog converter which provides a corresponding DC voltage on the basis of a setting which has been made.
Another preferred embodiment of the circuit arrangement according to the invention provides for the compensation voltage source to be a battery or a capacitor. The use of a capacitor is advantageous, in particular, since it is an extremely cost-effective compensation voltage source. In contrast, the disadvantage is that the capacitor is discharged relatively quickly again after a charging operation on account of leakage currents. Alternatively, provision is therefore made for the compensation voltage source to be a battery. Although the use of a battery is considerably more cost-intensive and less flexible than the capacitor, leakage currents have a lesser effect on the compensation voltage generated in contrast to the capacitor. A person skilled in the art can therefore adapt the circuit arrangement to the respective requirements through the selection of the compensation voltage source.
In the case of embodiments with a capacitor or a battery as the compensation voltage source, provision is particularly preferably made for the voltage recording device to also have a switch which bridges the voltage measuring unit. This makes it possible to very exactly set the DC voltage provided by the compensation voltage source to the open-circuit voltage of the test battery before the alternating current is applied, as provided for in another embodiment of the method according to the invention. This is done by first of all closing the switch and then opening it again in order to apply the alternating current to the test battery and record the voltage profile. As a result of the switch being closed, the test battery itself is used to charge the capacitor and to adapt the voltage of the compensation battery.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.